The existing reinforced concrete infrastructure (bridges, buildings, highways) in the U.S. and in many other countries is in bad condition due to cracking from the corrosion of the reinforcing bars. The early detection of the corrosion damage in reinforced concrete is quite challenging because the reinforcing bars are embedded in concrete; therefore, when the typical manifestations of corrosion distress become evident, stains on the concrete surface and cracking of the concrete structure, the reinforced concrete structure may be already seriously compromised.
The overriding cause of corrosion is the electrical potential between the metal and the solution. The rate of corrosion has been shown to be a very complex function of the metal solution interface, the reactive ions in the solution, the structure of the double layer, and the nature of reaction products which may form on the interface. The only direct measure of the state of the interface is the actual polarization voltage of the electrode formed at the interface with respect to a standard electrode. The measurement of this voltage forms the basis of several commercial systems designed to measure the corrosion potential and indirectly the corrosion rate. To make such a measurement, connection must be made to the reinforcing steel. This requires exposing the reinforcing steel which in itself is an operation which may lead to deterioration. Another approach to characterizing the metal solution interface which also requires connection to the reinforcing steel, is to determine impedance of the interface to current flow. If an alternating voltage is applied across a metal-solution interface current will flow and the voltage/current ratio is known as the Faradaic impedance. There is a relationship between the chemical state of the interface and the electrical impedance it offers to an impressed alternating current. The surface impedance, basically the resistance offered to the flow of current, is made up of two part, an energy absorbing or dissipative term which is called polarization resistance (Rp) and a capacitive component, C.sub.d1. The double interface layer between steel and concrete is basically a charge separation which expands and contracts under an impressed electric field and behaves electrically as a capacitance. The polarization resistance is a measure of the energy absorbed in the oxidation or reduction reactions occurring at the surface and the double layer capacitance reflects the structure of the double layer itself. The quantitative relationship is complicated by the formation of passive layers, the role of chlorides in the surface reactions, and the role of oxygen. Several studies have established useful qualitative relationship between the electrical equivalent circuit elements Rp and C.sub.d1 and the corrosion rate. Stearn and Geary, Stern, M. (1958) Corrosion, 14 pp 440, incorporated herein by reference, first showed the relationship between the polarization resistance Rp, and the corrosion rate. The result has been verified by many subsequent studies and the principle underlies a method of measuring the corrosion rate by contacting the rebar with one electrode and passing current to another electrode on the surface of the concrete.
For example, the rebar interface in well made concrete is coated with a passive layer or film of electrically insulating reaction products which presents a high polarization resistance and a capacitance dictated more by the thickness of the film than by the complex structure of the double layer adjacent to "bare" metal. During active corrosion this film is destroyed, the polarization resistance drops and the capacitance is set by the electrochemical properties of the interface. Further, the reaction or polarization resistance depends on frequency because the supply of ions to the surface is controlled by a diffusion law as well as by the mobility of the ions. Simple models of such diffusion controlled reactions show that the polarization resistance is frequency dependent and consequently complex. This frequency dependence is a function of the state of the interface, being quite different for the passive film or the active corrosion state.
In all of the processes responsible for corrosion there is an overriding requirement that the concrete must provide for relatively easy flow of ions, the electrical resistivity of the concrete must not be too high. Usually corrosion is initiated by carbonation which lowers the pH in the concrete or by the ingress of moisture, often accompanied by chlorides and permits ion dissolution to begin. Raising the moisture in the concrete also lowers it resistance so ion movement is facilitated and the corrosion rate increases.
A method to locate the reinforcing steel and measure the corrosion rate should have the following properties: It should from the surface, non-destructively, be able to locate the reinforcing steel and be able to localize the measurement so that portions of a steel undergoing corrosion can be distinguished from parts that are protected, measure the electrical resistivity of the concrete itself, and measure the properties of the interface, in particular the impedance because this is directly related to corrosion rate.